A unidirectional microphone is sensitive to sounds from only one direction. The sound intensity for a particular frequency is plotted for angles radially from 0 to 360°.

Describes a device that radiates or receives energy more efficiently from a single direction than from all other directions. In reference to microphones, unidirectional describes a polar pick-up pattern that more sensitive to sound from one direction than others. The actual polar pattern will dictate how directional microphone actually is.